Attention Deficit Hyperactivity Disorder (ADHD) is a genetically influenced brain disorder with a prevalence of 3-5% in school-age children. Affected sibling pairs and densely affected multi-generation families have been recruited from Colombia, South America, the US, and Germany. A genome-wide search for loci linked to ADHD has been completed and we have identified at least five regions in the human genome that contain genes to may contribute to ADHD (Arcos-Burgos et al. 2004). To date, we have complete clinical information on a total of over 600 small and large, multi-generation families, respectively (Palacio et al., 2004; Jain et al. 2006).